1. Field of the Invention
The present invention relates to a photoelectric conversion apparatus.
2. Description of the Related Art
FIG. 1 in Japanese Patent Application Laid-Open No. 2000-077644 illustrates a photoelectric conversion apparatus which uses a phototransistor. In the figure, a constant current source and a MOSFET driven by the constant current source constitute a common source circuit, and a voltage between a gate and a source of the MOSFET determines a potential of a base of the phototransistor. In the figure, the photoelectric conversion apparatus is structured so that a collector current of the phototransistor changes when a light quantity changes, and accordingly a voltage between the base and an emitter changes, but at this time, not the potential of the base of the phototransistor but the potential of the emitter thereof mainly changes. The photoelectric conversion apparatus makes the potential of the emitter biased by a larger electric current (to HFE×photo current) change instead of the base biased by the photo current, and thereby enhances the photo responsibility. Specifically, the photoelectric conversion apparatus shortens a period of time between the time when the light quantity has changed and the time when the changes of the potential of the base and the potential of the emitter are completed.
In a common source circuit in Japanese Patent Application Laid-Open No. 2000-077644, the voltage between the base and the emitter increases linearly with respect to an exponential increase of the light quantity. This is because a relationship of the following expression (1) holds between a voltage Vbe between the base and the emitter and a collector current Ic.Ic=Is×exp(qVbe/kT)  (1)
Here, Is represents a saturation current, q represents a quantum of electricity, k represents the Boltzmann constant, and T represents an absolute temperature. When the light quantity increases, the potential of the base slightly increases. Thereby, the output of the common source circuit in FIG. 1 in Japanese Patent Application Laid-Open No. 2000-077644 decreases, and thereby the potential of the emitter is lowered. Because of this, when the light quantity changes, some variation occurs in the potential of the base as well. This variation still results in determining the rate of the photo responsibility. This is because a mirror effect works on a capacitance between the base and the emitter (junction capacitance of phototransistor), and the capacitance seems to be a large capacitance. The common source circuit works as an inverting amplifier, and thereby the emitter varies in a direction opposite to the change of the base. Suppose that the gain of the amplification due to the inversion from the base to the emitter is 20, for instance. The capacitance between the base and the emitter contributes to the common source circuit as 20-times capacitance, and even though the potential variation of the base is small, the amount of an electric charge necessary for charge becomes large. The rate of the photo responsibility is thereby determined.